Understanding the Role of the First Carrier Bed: Simple Rules of Thumb and Workflows That Can Reduce the Dry Hole Rate
The first carrier bed is defined as the migration carrier adjacent to the source rock. It has the most significant control on lateral migration of petroleum and of charge access to all traps above it. Observation of oil and gas fields and dry holes and building migration models to account for them allow some simple rules of thumb, or models, to be devised and used for charge risking.
Globally, we find that success rates for wells targeting reservoirs within or adjacent to the first carrier bed/system are often above 80%. Examples include the Jurassic reservoirs of the North Sea, Arab formation in the Middle east, Silurian reservoirs of North Africa, the Paleogene of the deep Gulf of Mexico, the Norphlet play on and offshore GoM and many of the unconventional or hybrid petroleum systems. In fact, a significant fraction of world petroleum reserves is found in the first carrier bed.
Success rates drop exponentially with increasing distance above the first carrier because lateral migration creates “shadows” for targets above the first carrier. Charging of younger reservoirs is only possible where the relief of the first carrier allows migrating petroleum to form a column tall enough to exceed the capillary resistance to migrate vertically. This typically happens where the first carrier forms a three or four-way closure or a stratigraphic trap. Probability of charge for traps located directly above such features are significantly higher than average. Larger drainage areas allow the gathering of large volumes which mitigates the risk of the deeper section consuming all available volume (especially in basins with a lower quality or low maturity source). Large discoveries in the Tertiary/Cretaceous reservoirs of the North Sea, Miocene reservoirs of the deep water GoM and the most recent discoveries such as Zama and Liza prove this principle.
A useful rule of thumb is that charge access risk for shallow prospects is significantly reduced if the closure of the structure in the first carrier is > 500m in deep water marine systems or >150m in non-marine/deltaic systems. In most cases, we do not know a priori the capillary properties of the seal and there are significant uncertainties in extent and continuity of the first carrier and available expelled volumes. Hence, best practice is to assign a relative charge access risk based on the relief and drainage area of the first carrier. We will demonstrate the successful application of this approach.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019